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PhD Thesis
Direkte Messung und Bewertung des nebelgebundenen Eintrags von Wasser und Spurenstoffen in ein montanes Waldökosystem
Thomas Wrzesinsky (01/1999-02/2004)
Support: Otto Klemm
Direct Measurement and Evaluation of Fog Deposition of Water
and Trace Substances into a Mountainous Forest Ecosystem
Summary
For mountainous forests in Central Europe fog
deposition is likely to play an important role in the cycling of
water, nutrients, and pollutants. Quantifying the amount of occult
deposition was very difficult in the past due to technical
limitations. In this project a deposition measurement setup was
developed, tested and operated at the ecosystem research site
“Waldstein”. The system consisted of a fast droplet
spectrometer (measuring range from 1.5 to 50 µm diameter
in 40 size classes) and an ultrasonic anemometer to measure the
vertical wind component. From April 2001 through March 2002 a
continuous data set of fogwater deposition had been obtained. In
addition, visibility and the chemical composition of fog water (pH,
electrical conductivity, Na+, K+,
NH4+,
Mg2+, Ca2+, Cl–,
NO3–,
SO42– und PO43–)
were determined. Fog water was collected using an active heatable
cloudwater collector which was developed during this project. Samples
were taken every 8 h during fog events. The median sample volume was
249 ml. A total of 223 days with fog were observed during the
measuring period. The percentage of fog occurrence was 25.7 %.
Quality control of
data included stationarity and turbulence tests. Corrected data sets
were then used for flux calculations. During one year of measurements
(2001/2002) a total water input of
108 kg ha–1
by turbulent deposition and 17 kg ha–1 by
sedimentation was observed. Hence, turbulent deposition is dominating
the total fog precipitation of 125 mm with a percentage of
86 %.
Fogwater deposition shows a significant seasonal cycle. The highest
monthly amounts were observed during late fall and winter (24 mm
in January), lowest during the summertime (1 mm in August).
Measured droplet count spectra show peak values at diameters of 2, 6,
and 9 µm, respectively. The median droplet mass spectrum
has a maximum at 12 µm. Typical mass spectra have peak
values at 9, 12, or 15 µm diameter. For the liquid water
content the median is 156 mg m–3 with a
maximum of 2639 mg m–3 (5-min averages).
The largest contribution to total fogwater flux is in the droplet
diameter class from 14.5 to 15.5 µm. Droplets smaller than
7 µm are emitted (net balance), whereas larger droplets
are deposited. For the measuring period the amount of precipitation
by rain and snow was 1414 mm. Therefore, the percentage of
fogwater input amounts to about 8 % of the total input.
During the
measurements 253 samples of fog water were taken. For comparison, wet
only precipitation samples were taken on a weekly basis, too. The
chemical composition of fog and rain water is highly variable. For
fog water, median pH was 4.14, median concentrations were
621 µeq l–1
for NH4+,
487 µeq l–1
for NO3–, and
321 µeq l–1
for SO42–, respectively. These three
major ions contribute 87 % to the total concentration. Fogwater
concentrations are significantly higher than those in rain.
Enrichment factors are 18.1 (NH4+), 13.1
(NO3–), and 11.5
(SO42–),
respectively.
The fog deposition for
the main ions was calculated from concentrations and fogwater fluxes.
The inputs are 9.8 kg ha–1 for
NH4+
(7.9 kg ha–1 wet only),
27.9 kg ha–1
for NO3– (25.1) and
14.0 kg ha–1
for SO42– (15.0), respectively.
Therefore, occult deposition is a major contributor to total ion
inputs at that site, playing approximately the same role as rain and
snow precipitation. Nitrogen input is
13.9 kg N ha–1 a–1
(11.8 wet only). Throughfall measurements for the measuring period
show a nitrogen input of
23.3 kg N ha–1 a–1.
Hence, throughfall is nearly the same as the sum of fog, rain, and
snow. The role of dry deposition (e.g., the input of nitrate by
particles or gaseous HNO3) in the ecosystem balance has
not been quantified and has to be considered when trying to close the
nitrogen cycles as well as other nutrient fluxes. Another aspect is
the turnover within the tree canopy which may play a significant role
in closing the balance.
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